The invention relates to a method of blackening steel components, particularly, for cathode ray tubes. Examples of such components include shadow masks and shadow mask frames.
It is known from Valvo Berichte XVIII, 1/2 (1974) 84-87 that impinging electrons heat the shadow mask of color display tubes. An undesired spontaneous local expansion of the mask is often associated with the heating. This leads to so-called landing errors. These landing errors result in color defects since the electron beams no longer impinge upon the luminescent elements associated with them. Therefore, it is generally desired to keep the temperature of the mask low and as spatially constant as possible.
Several methods are known to keep the surface of the shadow mask cool. According to the above-mentioned Valvo Berichte, a mask is blackened by oxidation of its surface in an oxidation furnace. A black iron oxide is formed at the mask surface. Because of this blackening, effective radiation of the thermal energy produced by electron bombardment must be accomplished. This method has several disadvantages. The thermal emission power of such black layers depends upon the composition of the iron oxide formed which, in certain circumstances, may also become brown. Further, iron oxide in a vacuum tends to deteriorate under heating by electron bombardment. Both effects lead to a decrease of the thermal emission power. Finally, the iron oxide layers are heated by absorbing the electrons and their energy.
Providing blacker and simultaneously heat-withstanding layers on the mask surface is known from U.S. Pat. No. 3,867,207, which uses an electroless plating bath containing nickel compounds or cobalt compounds. The layer formed is subjected to a strong oxidation acid and then fired in air at approximately 450.degree. C. A disadvantage of the layers manufactured according to this method is that they also absorb the electrons and their energy. In addition, known plating baths comprise substances which are detrimental to health or even carcinogenic.
A mask coated with bismuth to reduce the passage of X-rays through the glass envelope of a color display tube is disclosed in U.S. Pat. No. 3,562,518. Blackening of the bismuth layer is not suggested. Also, it is not stated how the bismuth is provided on the shadow mask. According to Ullmanns Encyklopadie der technischen Chemie (Ullmann's Enyclopedia of Technical Chemistry), 3rd edition, vol. 18 (Munich-Berlin-Vienna 1967) p. 640, a bismuth coating can be provided on metallic articles by electrolysis, vapor deposition and electrodeposition. However, these methods have several disadvantages. For example, vapor deposition processes are technically expensive and require high investment while electrodeposition requires much energy and pollutes the environment.